The vacuum extractor is an obstetrical instrument, whose invention in 1849 is credited to Dr. James Young Simpson of Edinburgh (Scotland). It is comprised of a cup to be applied on the fetal head, of a tubing extending from the cup to a vacuum pump, and of a traction handle. As for the use of this instrument, once the cup is correctly applied to the fetal head (over the sagittal suture, with the posterior margin 3 cm forward of the posterior fontanelle, so that the center of the cup is over the flexion point of the fetal head), a vacuum pressure is generated under the cup with a hand pump or an electric pump. When the negative pressure within the cup reaches 550-600 mm Hg, traction is begun by pulling on the traction handle with the right hand perpendicular to the cup, while the left hand presses on the cup and the fetal head posteriorly, thus maintaining flexion of the head and the vector force follows the pelvic axis, which is the one of less resistance. Because the purpose of the vacuum is to augment the natural forces of labor, and not to replace them, traction is best applied in concomitance with the uterine contractions, for the so-called “push-pull effect.”
The vacuum extractor is very valuable to shorten or to end the second stage of labor, whenever to do so is in the best interest of the mother or the fetus. Maternal indications for the use of the vacuum extractor include medical problems that would make the patient a poor candidate for a cesarean section, such as disabling cardio-pulmonary disease (mitral stenosis, congestive heart failure) or conditions that would worsen with excessive pushing, such as retinal detachment, or simple exhaustion from prolonged labor or protracted expulsive efforts. Fetal indications include conditions that expose the fetus to imminent danger of death, such as prolapse of the umbilical cord, bleeding from vasa previa, premature placenta separation, or a worrisome fetal heart pattern, and an emergency cesarean section cannot be done in a timely fashion.
The vacuum extractor requires less dexterity and thus, it is perceived as having greater safety, due to the decrease in compressive force, when compared to forceps. This may help explain why its use has increased from 3.5% to 5.9% of all deliveries from 1989 to 1995. However, the vacuum extractor is known to cause numerous fetal injuries, including scalp abrasive and ecchymotic lesions, and the more severe cephalohematomas (i.e., separation of the scalp from the underlying structures), subaponeurotic (a.k.a. subgalean) hemorrhage (i.e., collection of blood in the potential space between the cranial periosteum and the epicranial aponeurosis), intracranial hemorrhage (subdural, subarachnoid, intraventricular, and/or intraparenchimal), and retinal hemorrhage.
The incidence of the above fetal injuries can be reduced by limiting the negative pressure under the cup to 550-600 mm Hg, by keeping the cup on the fetal scalp for no longer than 15 minutes, and by avoiding cup “pop-offs.” However, whereas the vacuum pressure and the time of cup application can be objectively determined, how to avoid cup “pop-offs” is left to the judgment of the obstetrician, who must learn from experience how much traction can be exerted before they happen. Consequently, detachment of the cup is very common, particularly in the hands of the novice, due to the general tendency to inadvertently apply a traction force that exceeds the adhesive force of the cup. Unfortunately, cup “pop-offs” expose the fetus not just to scalp abrasive and ecchymotic lesions, but potentially to the more severe injuries mentioned above, given the tendency in such instances to increase the negative pressure under the cup, in order to be able to apply a greater traction force.
Studies have been conducted to determine the detachment force, i.e., the maximum traction that the obstetrician can apply during a vacuum-assisted delivery without causing a cup “pop-off.” Unfortunately, the instruments used to determine the detachment force of the various cups, while useful for research purposes, are not well suited for routine clinical use. Examples of such devices include those briefly described in the scientific literature by Moolgaoker A. S., et al., Mishell D., et al., Duchon M. A., et al, and Hofmeyr G. J., et al.
To the applicant's knowledge, the only vacuum extractor device presently in clinical use, in this country, with the means to measure the traction force, is the Kiwi vacuum delivery device (Clinical Innovations Inc., Murray, Utah 84123). However, this instrument has two serious drawbacks. The first is that the traction force during the vacuum-assisted delivery is measured through a mechanical gauge, and thus of limited precision. The second drawback is that in order to control the traction force that is being exerted on the fetal scalp, the doctor must divert the attention from the fetal contact cup to the small scale on the traction force indicator.
Accordingly, there is a need for an improved obstetrical vacuum extractor that can measure the traction forces applied to the fetal head without the constraints and design drawbacks seen in the prior art and described above.